A conventional electromagnetically driven type plunger pump of a non-powering feed type comprises, for example, a plunger which is disposed in a cylinder (a cylindrical body) and being free to reciprocate, a pair of springs which exert specific urging force to the plunger from both ends thereof while having consistent contact therewith, a solenoid coil which exerts thrust (electromagnetic force) to the plunger to suck a liquid, a magnetic circuit including a yoke and various check valves.
The pair of springs is disposed to have consistent contact with the plunger, and dampen a vibration of the plunger while retaining it at a specific resting position during a non-powered resting state with energy of the springs released, or perform together as feeding springs to accumulate energy for feeding.
Further, as shown in FIG. 7, the thrust (electromagnetic force) generated by the magnetic circuit has a characteristic that it is maximum when plunger 3, which is urged by pair of springs 2, is located at the vicinity of yoke 1 which forms the magnetic circuit. In other words, this obtained thrust shows a mountain-shaped characteristic as being small in an early range and a later range, and being large in a middle range.
Additionally, as shown in FIG. 8, with the electromagnetically driven type plunger pump, a threshold F0, which is determined by a target discharging pressure (a feeding pressure) and a diameter (area) of the plunger, is present. Here, the plunger 3 cannot be moved towards a feeding direction when the urging force of spring 2 does not exceed the threshold F0.
On the other hand, as shown in FIG. 8 with a two-dot chain line, it is ideal to obtain an effective stroke Si as large as possible with spring constant ki of the spring 2 set relatively small, so that a feeding liquid amount (discharged amount) is increased with a moving stroke of the plunger 3 increased as large as possible. However, in this case, as shown in FIG. 8 with oblique lines, the urging force of the spring 2 exceeds the thrust in the early range. Consequently, even if power is supplied during a sucking process, the plunger 3 cannot be operated and compression of the spring 2, namely accumulation of the energy, is not performed.
Therefore, as shown in FIG. 8, when a liquid discharging pressure (feeding pressure) is set relatively high (200 kPa˜300 kPa, for example), and also with restrictions of size of a product and the like, spring constant k of the spring 2 is set relatively large, resulting in that effective stroke S of the plunger 3 is small. Accordingly, a discharging amount (feeding amount) cannot be increased, and an increase of power consumption or upsizing of the solenoid coil is needed to obtain a necessary discharging amount.
The present invention is accomplished in the light of the above-mentioned points, and a purpose is to provide an electromagnetically driven type plunger pump which has a highly efficient discharging (feeding) performance with an effective stroke of the plunger being large, while being in a quest of simplifying structure, downsizing, reducing power consumption, reducing noise, and the like.